1. Field of the Invention
This invention relates generally to imaging detectors, and more particularly, to a photosensing circuit which provides a differential output substantially free of electrical noise.
2. Description of Related Art
Imaging detectors are usually formed by arranging single photodetectors in linear one-dimensional or two-dimensional arrays. The outputs of the photodetectors are connected in parallel to the processing circuits. Since the output of each of the photodetectors may travel a considerable distance to the processing circuits, electrical noise may be induced from current circulating in the power rails and neighboring circuits, and may be picked up by the output lead and added to the photodetector output signal. Electrical noise may also originate from injected noise in the power sources which is directly delivered to the photodetectors via the power rails. Electrical noise may also be inherent, caused by fabrication defects due to tightly packed clusters of circuits. Electrical noise in a photodetector, whether induced, injected or inherent, is common-mode in nature. This electrical noise may be substantially greater than the photodetector output signal, and is one of the most undesirable characteristics of the currently available complementary metal-oxide silicon (CMOS) photodetectors. Presently, this electrical noise is estimated and compensated for in the processing circuits.
Thus, currently, there is a need for a photosensing circuit which has an inherent property of rejecting induced, injected or inherent electrical noise from power rails and neighboring circuits. Such circuit would eliminate the need to compensate for the induced, injected and inherent electrical noise in the processing circuits. The output of such a circuit would accurately represent the amount of light impinging on a photodetector and could be directly processed.